The present invention relates to acoustic borehole logging tools having dipole transmitters. In particular, the invention relates to the use of a novel dipole transmitter for use in a borehole logging tool which addresses certain shortcomings of prior art designs.
The field of sonic logging of boreholes in the oil and gas industry involves making acoustic measurements in the borehole at frequencies typically in the range 500 Hz-20 kHz. Below this range is typically considered as the seismic domain, above it the ultrasonic domain. In some cases, but not all, techniques and technologies are transferable between these domains. A summary of the general techniques involved in borehole acoustic logging can be found in GEOHYSICAL PROSPECTING USING SONICS AND ULTRASONICS, Wiley Encyclopaedia of Electrical and Electronic Engineering 1999, pp 340-365.
One example of a sonic logging tool is Schlumberger""s Dipole Sonic Imaging tool (DSI) which is shown in schematic form in FIG. 1. The DSI tool comprises a transmitter section 10 having a pair of (upper and lower) dipole sources 12 arranged orthogonally in the radial plane and a monopole source 14. A sonic isolation joint 16 connects the transmitter section 10 to a receiver section 18 which contains an array of eight spaced receiver stations, each containing two hydrophone pairs, one oriented in line with one of the dipole sources, the other with the orthogonal source. An electronics cartridge 20 is connected at the top of the receiver section 18 and allows communication between the tool and a control unit 22 located at the surface via an electric cable 24. With such a tool it is possible to make both monopole and dipole measurements. The DSI tool has several data acquisition operating modes, any of which may be combined to acquire (digitised) waveforms. The modes are: upper and lower dipole modes (UDP, LDP)xe2x80x94waveforms recorded from receiver pairs aligned with the respective dipole source used to generate the signal; crossed dipole modexe2x80x94waveforms recorded from each receiver pair for firings of the in-line and crossed dipole source; Stoneley modexe2x80x94monopole waveforms from low frequency firing of the monopole source; P and S mode (PandS)xe2x80x94monopole waveforms from high frequency firing of the monpole transmitter; and first motion modexe2x80x94monopole threshold crossing data from high frequency firing of the monopole source.
Various types of dipole signal source have been proposed and used in the past. These include:
i) Electro-magnetic transducer devices such as is used in Schlumberger""s DSI tool. (see for example Hoyle et al; U.S. Pat. No. 4,862,991 and Kitsunezaki in U.S. Pat. No. 4,207,961 or Ogura in U.S. Pat. No. 4,383,591).
ii) Linked mass vibrators driven by magnetostricitve actuators. (see for example Cohick and Butler, xe2x80x9cRare-earth Iron Square Ring Dipole Transducerxe2x80x9d, J. Acoustical Society of America, 72(2), August, 1982)
iii) Piezo-electric bender devices such as are used in the XMAC tool of Baker Atlas. (see for example Angona et al, U.S. Pat. No. 4,649,525)
iv) Magnetic repulsion transducers driving a plate in contact with a fluid in an acoustic wave guide system such as are used in the MPI XACT tool. (see for example Gill et al, U.S. Pat. No. 5,852,262)
v) Eccentric orbital masses as proposed by Cole in U.S. Pat. No. 4,709,362, Meynier in U.S. Pat. No. 5,135,072 and others, mainly for seismic uses.
Dipole sonic sources of types i) to iv) described above typically comprise a heavy, stiff tool body having the actuator (piston or plate) mounted therein via a transducer or drive mechanism, the actuator contacting the borehole fluid through ports in the tool body. In use, the tool body acts a reaction mass against which the transducer acts to oscillate the actuator. However, the effect of this is to excite tool body recoil vibrations, which interfere with the dipole flexural signal in the borehole. In these sources the tool body is used as the reaction mass. Its large vibrating surface is a very efficient radiator of noise into the borehole flexural mode. This means also that increasing the excitation force increases recoil vibrations in the same proportion as the signal. Another problem is that the dipole signals couple into the tool structure and travel along it directly to the receivers where they interfere with the detection of the signals of interest from the formation. Various measures have been used or proposed to deal with this problem, for example: locating the source and receivers in separate sondes connected by a flexible cable; the use of isolation joints which include structures for attenuating or delaying signals travelling along the tool; or adopting a structure which does not include any continuous mechanical structure along the length of the tool so as not to provide a signal path; or the use of housings around the receivers which delay the arrival of tool signals. Transmitters of these types are imperfect dipoles, because of the limited azimuthal extent of the active radiating surface (i.e. the ports in the tool body). Strong hexapole aliasing can be produced by such sources, and possibly strong monopole contamination when the source is eccentered in the borehole. Furthermore, bender (piezoceramic bimorph) sources (type iii) are inherently band limited in frequency, and radiate over a small azimuthal extent.
Orbital vibrators and counter-rotating eccentric mass devices (type v) are typically appropriate only for lower frequencies, typically below 500 Hz., and are often found in seismic applications. Such sources are not normally considered as suitable for broad band or higher frequency use such as is encountered in sonic logging.
If the full flexural dispersion curve (phase slowness vs. frequency) is to be used to make measurements at different depths of investigation for a wide variety of petrophysical, geophysical, and geomechanical applications, wideband dipole sources of high purity will be required. Moreover, in dipole tools, where the flexural impedance of even the most xe2x80x9crigidxe2x80x9d tool body is low, recoil vibrations of the tool body, wave propagation along the tool body, and reflections at tool joints and within acoustic isolation sections, have all been found to radiate noise into the borehole and formation, contaminating the flexural signal. Consequently, the transmitter and tool body should be designed as a system to minimize these noise sources.
The present invention provides a logging tool comprising a tool body, which can be positioned in a fluid-filled borehole, having a receiver section and a dipole transmitter; wherein the dipole transmitter includes a transducer comprising a shell having a reaction mass and a motor located therein, the motor operatively connecting the shell and the reaction mass such that only an outer surface of the shell is in contact with the fluid in the borehole.
The present invention concerns a new type of dipole source for well logging. Specifically, the new source involves the idea of shaking all or part (axially) of a dipole tool body to produce a pure, broadband acoustic dipole signal while at the same time coupling as little energy as possible into the tool body. Important variations on this idea include a linear phased array of shaker sources, and active cancellation of tool borne noise.
The dipole transducer is preferably mounted in the tool body by means of spring mountings, the resonant frequency of which is such that coupling of flexural vibrations from the receiver to the tool body in a predetermined frequency range is inhibited. Typically, the resonant frequency of the spring mountings is less than the lower limit of the predetermined frequency range. It is also preferred that the spring mountings are relatively flexible in the direction of oscillation of the dipole transmitter and relatively stiff in a direction orthogonal thereto.
The resonant frequency of the shell also preferably falls outside a predetermined frequency range, and the weight of the shell is less than the weight of the reaction mass.
The dipole transducer can be located in a cavity in the tool body, structural members extending between parts of the tool body on either side of the cavity around the periphery of the cavity. The resonant frequency of the structural members preferably falls outside the frequency range of interest.
Alternatively, where there is a structural load bearing member running along the length of the tool inside the tool body, the dipole transducer can be mounted on the load bearing member.
Preferably, the shell is shorter than the shortest wavelength of acoustic waves in the formation surrounding the borehole to be measured. In any event, it is desirable to have the shell as short as possible so as to resemble closely a point source.
In an embodiment of the invention, a stiff (in the frequency range of interest) shell directly coupled to the borehole fluid is excited in the radial sense relative to a heavy internal reaction mass by an appropriate linear motor. This source produces a purer dipole signal than sources consisting of active elements (pistons, benders, hydraulic outlets) of small azimuthal extent located on heavy tool bodies, less hexapole being produced due to the full azimuthal extent of the radiating surface. In this embodiment, recoil vibrations of the reaction mass are prevented from coupling to the borehole liquid, where they would otherwise produce interfering noise in the borehole flexural signal.
The transmitter should produce little or no monopole component, whereas prior art transmitters produce both monopole and dipole. The external shell is typically cylindrical, but other shapes (spherical, ovoid, etc.) are possible. The shell can be either stiff in bending (having no bending modes in the frequency range of interest), or alternatively, the first bending mode of the shell can be excited deliberately to amplify the output. This embodiment will produce a more narrow band output than the rigid shell configurations. Given the extreme pressures encountered in the borehole environment, the shell must be either pressure balanced, or strong enough to resist absolute pressure. In the case of a liquid filled, pressure balanced shell, sufficient radial clearance must be maintained between the inside of the shell and the outside of the reaction mass to minimize the resistive loading due to viscous losses in the liquid (oil). An air filled shell must be thick and strong enough to withstand absolute pressure (typically 15-20 kpsi), however clearances between the shell and the reaction mass can be made smaller than in the oil-filled case. Since keeping resonant modes of the shell out of the frequency range of interest typically requires a fairly thick shell, a relatively thick, air filled shell is one preferred embodiment for the source.
With an air-filled shell the external support/suspension which connects the transducer to the tool body will typically be in direct contact with the radiating shell. The oscillations of the shell will directly excite acoustic vibrations in the support. In an oil-filled embodiment, the support/suspension can be made to contact only the internal reaction mass. Because the oscillations of the heavy reaction mass are typically of much smaller amplitude that those of the lighter shell, vibrations transmitted to the support will be of correspondingly smaller amplitude than in the air filled case. In another preferred embodiment, the source is configured to produce output in both the X and Y (orthogonal) directions at one axial level. In another embodiment, X and Y sources are located at different, but closely spaced axial levels. Transmitters can be mounted in slotted housings and in the form of arrays of multiple transmitters operated as a phased-array if required.
In a phased-array configuration, a length of source housing can be excited by multiple motors so as to match the excitation in amplitude and phase of a given section of the borehole (i.e. a single shell). Alternatively, an axial array of rigid shells can be excited with appropriate amplitude and phase to match the excitation function of a given borehole. These embodiments with more than one independent motor are considered to be phased array dipole sources.
In the case where the shell comprises part of a tool housing also containing receiving elements at another axial location, appropriate control of the excitation of the driven section can be performed to prevent flexural vibrations from propagating into the receiver section. This case combines both source and active tool noise reduction functions.
A typical frequency range for present dipole sonic logging in the borehole is 0.5-5 kHz. For next generation tools it is of interest to extend this range to lower frequencies if the source is pure enough not to excite Stoneley waves. It is also of interest to extend this range upward to 10 kHz. Orbital vibrators are appropriate only for lower frequencies, typically below 500 Hz.
For the above stated frequencies the linear motor (actuator) technologies listed hereafter are considered appropriate:
Electrodynamic (moving coilxe2x80x94loudspeaker type)
Electromagnetic (magnetic attractionxe2x80x94BBN type)
Magnetic repulsion (Lorentz force or eddy current)
Piezoelectric stack
Magnetostrictive bar
Hydraulic (high frequency servovalve)
Hybrid (piezo-hydraulic, piezo with mechanical amplification, etc. Rotating eccentric mass systems currently available are generally not considered appropriate above 500 Hz but may be applicable for frequencies at or below this limit.